The present invention relates to an ultrasonic flowmeter which operates in accordance with the propagation time difference method.
The propagation time difference method is known as a method for measuring a flow rate (flow velocity or flow volume) of a fluid such as a gas or a liquid by utilizing an ultrasonic flowmeter. As shown in FIG. 1, a pair of ultrasonic transducers 101 and 102 are mounted in a conduit 100 such that they oppose each other on a line which is inclined by an angle .theta. with respect to a fluid flowing direction in the conduit 100. A flow rate, such as the flow velocity of a fluid, is measured in accordance with a propagation time difference .DELTA.T between the ultrasonic propagation time Td in a downstream direction (i.e., the time required for an ultrasonic wave to propagate from the ultrasonic transducer 101 to the ultrasonic transducer 102) and ultrasonic propagation time Tu in an upstream direction (i.e., the time required for an ultrasonic wave to propagate from the ultrasonic transducer 102 to the ultrasonic transducer 101).
The propagation times Td and Tu are given as follows: EQU Td=2l/C+L/(C+Vcos .theta.) (1) EQU Tu=2l/C+L(C-Vcos .theta.) (2)
where
l is the length of a region (recess) 103 or 104 which is free from the influence of the flow velocity of the fluid between the ultrasonic transducers 101 and 102, PA1 L is the length of a region which is influenced by the flow velocity of the fluid between the ultrasonic transducers 101 and 102, PA1 V is the flow velocity of the fluid, PA1 .theta. is the angle formed between a direction indicated by an arrow 105 and a line (indicated by a dotted line) connecting the ultrasonic transducers 101 and 102, and PA1 C is the sonic velocity in a fluid at rest.
The propagation time difference .DELTA.T is thus given as follows: ##EQU1## In general, the relation V&lt;&lt;C is given, so that the following relation is established: EQU C.sup.2 .apprxeq.C.sup.2 -V.sup.2 cos.sup.2 .theta. (4)
The flow velocity V of the fluid is given as follows: EQU V=(C.sup.2 /2L cos .theta.).multidot..DELTA.T (5)
In this manner, when the propagation time difference .DELTA.T is measured, the flow velocity V can be obtained in accordance with equation (5).
The propagation time difference .DELTA.T is generally as small as 10.sup.-4 times the propagation time Td or Tu. For example, when the conduit 100 shown in FIG. 1 comprises a water pipe which has a diameter of 100 mm, and the flow velocity V is 1 m/sec, the propagation time difference .DELTA.T is about 150 nsec. Furthermore, when the conduit 100 comprises a gas pipe which has a diameter of 10 mm, and the fluid velocity is 10 cm/sec, the propagation time difference .DELTA.T is about 27 nsec.
It is normally rather difficult to precisely measure a very small propagation time difference .DELTA.T. In the flow meter of this type, the ultrasonic transducers 101 and 102 are simultaneously driven by separate drivers, respectively, so as to decrease crosstalk components received by the ultrasonic transducers 101 and 102. A time difference is measured as the propagation time difference .DELTA.T when the ultrasonic wave from the ultrasonic transducer 101 is received by the ultrasonic transducer 102 and vice versa. The drive timings of the ultrasonic transducers 101 and 102 may be different (corresponding to a difference .DELTA.t) by several nanoseconds to several tens of nanoseconds in accordance with a difference between the switching characteristics (i.e. a difference between the rise and decay times of the transistors) and a difference between lead wires connecting the ultrasonic transducers 101 and 102 with their corresponding drivers. The difference .DELTA.t is actually measured as part of the propagation time difference .DELTA.T, so that an offset in the measured value for the flow velocity V occurs.
In the flowmeter of this type, the arrival timings of the ultrasonic waves from the ultrasonic transducer 101 at the ultrasonic transducer 102 and vice versa are determined in accordance with an Nth ultrasonic wave (where N corresponds, for example, to the wave which has a maximum magnitude). In this condition, when frequencies f1 and f2 of the ultrasonic waves respectively produced from the ultrasonic transducers 101 and 102 do not coincide, a frequency difference is produced as an arrival timing difference .DELTA.t'. The difference .DELTA.t' is given as N(1/f1-1/f2). For example, if the frequencies f1 and f2 are 1.00 MHz and 1.01 MHz, respectively, and N is 5, the difference .DELTA.t' is about 50 nsec. The difference .DELTA.t' results in an offset in the measured value in the same manner as does the difference .DELTA.t.
Conventionally, the offset value caused by the differences .DELTA.t and .DELTA.t' is eliminated in the following manner. The ultrasonic transducers 101 and 102 are driven when the fluid is interrupted in the conduit 100. In this condition, an offset is measured in accordance with equations (3) and (5). In subsequent empirical measurement, the offset previously measured is subtracted from the measured value of the flow velocity V obtained in accordance with equation (5). Thus, an actual value of the flow velocity V is obtained.
However, the fluid must be interrupted during the offset measurement. For this purpose, the conduit 100 must be removed from the corresponding piping, or the fluid supply to the conduit 100 must be stopped. Therefore, the offset elimination method of the type described above is difficult to apply with a flowmeter which is operating continuously. Furthermore, a change in offset over time cannot be precisely measured.